Choline is a precursor for the biosyntheses of both the choline-phospholipids and the neurotransmitter acetylcholine. Dietary sources provide a large part of the humans daily choline requirement, and variations in diet can alter the availability of choline to tissues. Infant mammals derive all, or most of their nutrient from milk (which contains free choline as well as the choline-containing phospholipids lecithin and sphingomyelin) and, because they are growing rapidly, they require large amounts of choline for membrand biosynthesis. Little is known about the mechanisms which accumulate choline within mikl. Preliminary studies demonstrate that milk choline content changes during development, and is influenced by the maternal diet. Studies are proposed which will examine the factors which influence the choline and methionine content of milk (e.g., variations due to developmental, diurnal, dietary and sampling factors), the mechanisms for transport of choline into milk, and the effects of diet upon tissue choline concentrations.